Crystalline polypropylene, which has superior characteristics such as good mechanical properties and high chemical resistance, has been widely used in various plastic processing fields. Propylene homopolymers and random copolymers thereof with small amounts of α-olefins have high rigidity, but some of these polymers may have insufficient impact resistance.
Accordingly, attempts have been made to improve the impact resistance, including the addition of a rubber component such as an ethylene-propylene rubber (EPR) to a propylene homopolymer and the manufacture of an impact copolymer containing a rubber component through homopolymerization of propylene and subsequent copolymerization of propylene with ethylene or α-olefin. An increase in the content of the rubber component can improve the flexibility and impact resistance of the impact copolymer.
Another problem exists in that an impact copolymer prepared in the presence of a traditional Ziegler-Natta catalyst inevitably contains low-molecular-weight components (e.g., oligomers). According to recent trends, the impact copolymers have high flowability for further improvements in moldability.
Unfortunately, excess flowability of the rubber component results in the generation of larger amounts of low-molecular-weight components, which cause various problems, such as fumes and odors during processing, detrimental effects on smell and taste after processing, and promoted blocking due to high stickiness. A polymer with poor powder characteristics cannot be stably manufactured. A larger difference in average molecular weight between the crystalline polypropylene and the rubber component causes problems such as a high gel content in a molded product and a high linear expansion coefficient of the molded product.
Metallocene catalysts, which are different from traditional Ziegler-Natta catalysts, are known for use in the polymerization of propylene into isotactic polypropylene.
Similar catalysts are also known for use in the manufacture of impact copolymers through homopolymerization of propylene and subsequent copolymerization of propylene with ethylene (see, for example, PTLs 1 and 2). Also disclosed are impact copolymers having satisfactory rigidity and high impact resistance (see, for example, PTL 3).
To achieve high impact resistance, impact copolymers must have a lower glass transition temperature, for example. To satisfy this requirement, it is preferred to copolymerize propylene with ethylene or α-olefin such that their respective contents fall within certain ranges (see, for example, NPL 1).
Many transition metal compounds are known for use as components of metallocene catalysts. Also known are transition metal compounds that produce homopolypropylene having a high melting point to provide an impact copolymer with improved rigidity (see, for example, PTL 4).
Unfortunately, the manufacture of such an impact propylene copolymer in the presence of a metallocene catalyst has the following technical problems associated with the difference in reactivity between propylene and other comonomers.
Specifically, the copolymerization of propylene with ethylene or α-olefin after homopolymerization of propylene in the presence of a metallocene catalyst by a conventional process may result in a large difference between the ratio of propylene to ethylene or α-olefin contained in the gaseous composition in the polymerization atmosphere and the ratio of propylene units to ethylene or α-olefin units in the copolymer polymerized in that atmosphere. This may result in a smaller amount of ethylene or α-olefin component in the polymer. To manufacture a copolymer having a desired ethylene or α-olefin content, the gaseous monomers must be supplied and polymerized in a ratio that differs largely from that of the copolymer. Such control is disadvantageous for manufacture. In extreme cases, a copolymer having a desired ethylene or α-olefin content cannot be manufactured because of the restraints imposed by polymerization systems.
Accordingly, it is desirable to develop a method for manufacturing polypropylene at a high uptake rate of ethylene and α-olefin in the presence of a metallocene complex catalyst without involving a large difference in ethylene content between the gaseous ethylene/propylene mixture and the resulting polymer.
Another problem associated with traditional metallocene catalysts is production of a copolymer having a low molecular weight in gas-phase copolymerization of propylene with ethylene or α-olefin. To provide a propylene-ethylene block copolymer with high impact resistance, the resulting copolymer must also have a certain molecular weight. Accordingly, development of a method for manufacturing a copolymer having a high molecular weight is eagerly awaited. Also awaited is development of a catalyst having high rubber polymerization activity in order to decrease the catalyst cost per unit polymer and increase the rubber content.
As described above, homopolypropylene having a high melting point is required to provide an impact copolymer with improved rigidity. Unfortunately, among the above metallocene catalysts that have an improved uptake rate of ethylene and α-olefin and facilitate manufacture of a copolymer having a high molecular weight, a metallocene complex catalyst is yet to be known that sufficiently functions as a catalyst for manufacturing homopolypropylene having a high melting point.
PTL 5 discloses a metallocene complex having a substituent at position 5 of one or each indenyl ring and an optionally substituted furyl or thienyl group at position 2 of one or each indenyl ring. This metallocene complex has a high ethylene uptake rate and can provide a copolymer having a high molecular weight.
PTL 6 discloses an asymmetric metallocene complex having methyl groups at positions 5 and 6 and an alkyl group at position 2 of one of the indenyl rings. This metallocene complex has a high ethylene uptake rate and can provide a copolymer having a high molecular weight.
Unfortunately, the metallocene complexes disclosed in PTLs 5 and 6 are still unsatisfactory because they cannot produce homopolypropylene having a sufficiently high melting point. Accordingly, development of a metallocene complex with improved performance is eagerly awaited.